I was working on a small 30sec commercial that someone handed off to me which was mainly graphics and VO when I noticed that every time I rendered it a seemingly random and different each time frame was just full screen green.
Usually QuickTime does this when it can "read" a file or codec or something along those lines. Checking the timeline I saw that the background layer, the only thing to go green in a bad way, was a .mpg, specifically MPEG2 Muxed. Quicktime seemed to have an issue with it either rendering in the timeline or exporting to a QT Movie.
So, I tossed the offending .mpg into QT Player, exported it as a DV stream (this wasn't HD, obviously) imported it into FCP and replaced the old .mpg with it in the timeline and all was well after re-rendering and re-exporting.
The lesson here is despite what Apple and the FCP manual's tell you highly compressed codecs aren't the best thing to work with if there is a lesser compressed choice.
Yes yes, I'm aware of generational lossy-ness...